By comparing the organization, specificity and function of regulatory elements in Salmonella phage P22 and coliphage lambda, we have gained an insight into both the regulation and the evolution of temperate bacterial viruses. I propose that a second Salmonella phage, temperate phage L, now be studied with respect to both phage P22 and the lambdoid phage family. To facilitate the physical and functional analysis of phage P22, L and lambda the regulatory regions of the P22 and L genomes will be incorporated into the lambda genome (occurs in vivo). The construction and characterization of a variety of lambda-P22 hybrids has already been reported. A set of lambda-L hybrids will be isolated and their structure determined from genetic and DNA heteroduplex mapping. Next a physical map of P22 and L will be generated from heteroduplex analysis of lambda-P22, lambda-L and lambda DNA. Using physical markers of known size and location and mutations in P22 and L genes, the genetic and physical maps of these phages will be correlated. Recombination occurring between P22 and L genes and within P22 and L genes will be measured and the region in which recombination occurred visualized and verified by DNA heteroduplex analysis. From the study of these recombinant phages, we will also learn what new combinations of P22, L and lambda regulatory elements are viable and in what ways we can manipulate prokaryotic gene expression.